Optical fibers are well known in the art and useful for many applications in modern communications systems. A typical fiber optic communications system, for example, is shown schematically in FIG. 1A. The system comprises a source of optical signals 10, a length of optical fiber 12 coupled to the source, and a receiver 14 coupled to the fiber for receiving the signals. One or more amplifying systems 16a, 16b, may be disposed along the fiber for amplifying the transmitted signal. Filters and attenuators are useful in these systems to change the power levels of various signals, especially in wavelength division multiplexed systems, along with signal modulation and wavelength routing.
Basically, the optical fiber 12 shown in FIG. 1A comprises an inner core fabricated from a dielectric material having a certain index of refraction, and a cladding surrounding the core. The cladding is comprised of a material having a lower index of refraction than the core. As long as the refractive index of the core exceeds that of the cladding, a light beam propagated along the core exhibits total internal reflection, and it is guided along the length of the core.
Since the conventional optical fiber confines most of the light in the core region, with such conventional fibers the ability to effect propagation behavior of the light in the fiber core is significantly limited. With conventional fibers, to change the propagation behavior of light in the core or in attenuating the signal, one is limited in the configurations that may be used, usually to use of temperature and/or strain. For example, U.S. Pat. No. 5,321,790 to Takahashi, et al., "Optical Attenuator Using an Optical Fiber and Method and Apparatus for Producing Same," issued Jun. 14, 1994 and incorporated herein, shows an attenuator formed by heating the fiber with electrodes or gas burners and compressing the fiber in the axial direction to provide a light attenuating portion along the fiber. A low reflection attenuation device for use in an optical fiber connector is described in U.S. Pat. No. 5,082,345 issued to Cammons el al. on Jan. 21, 1992, entitled "Optical Fiber Connecting Device Including Attenuator" (the "Cammons patent") assigned to the assignee herein, which is hereby incorporated by reference. The Cammons patent describes use of polymethylmethacrylate (PMMA) to fabricate a disc-shaped attenuator portion disposed at the end of the transmission path or within an optical fiber connector. The attenuator portion produces -40 dB reflectance which is suitable for many applications but less than optimal for high performance optical fiber systems.
Variable attenuators, i.e., in which the degree of attenuation is controlled, typically have comprised complicated structures with moving parts that rotate or otherwise move the position of the fiber or attenuator. For example, U.S. Pat. No. 5,745,634 to Garrett, et al., "Voltage Controlled Attenuator," issued Apr. 28, 1998 and incorporated herein, shows a variable attenuator with which the variation in the attenuation is obtained by actuating a dc motor which displaces the position of the attenuator. Similarly, U.S. Pat. No. 5,677,977 to Smith, "Optical Attenuator," issued Oct. 14, 1997 and incorporated herein, shows a variable attenuator with which the variation in the attenuation is obtained by providing a circular loop of optical fiber which is rotated with use of a lockable rotating shaft clamped to the side of the loop. U.S. Pat. No. 5,781,341 to Lee, "Motorized Tunable Filter and Motorized Variable Attenuator," issued Jul. 14, 1998 and incorporated herein, shows a variable attenuator with use of a cam attached to a collimator; the cam rotates the collimator to adjust the insertion loss. U.S. Pat. No. 5,319,733 to Emmons et al., "Variable Fiber Optical Attenuator," issued Jun. 7, 1994 and incorporated herein, shows a variable attenuator with use of two terminated fibers that are placed in holdings with their terminal ends aligned; the holders are rotated relative to each other while the alignment is maintained to provide variable attenuation. As can be seen, each of these variable attenuators involve use of moving parts.
As may be appreciated, those concerned with the development of optical communications systems and more particularly, fiber devices, continually search for new components and designs including new attenuator designs. As optical communications systems become more advanced, there is growing interest in increasing the number of wavelengths that may be transmitted by the systems and therefore in new methods and devices for modulating, filtering, and switching wavelength channels. The instant invention provides a new structure for an optical fiber device and in particular, a variable attenuator device that involves no moving parts.